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Conservation > Fish & Wildlife Diseases Echinococcus

For questions/concerns about this disease in humans, please call your doctor or the Montana Department of Public Health and Human Services  (DPHHS).

For questions about this disease/parasite in wildlife, please call the FWP Wildlife Health Lab at (406) 577-7882.

Cause

Echinococcus is a genus of tapeworm. Two species of Echinococcus (Echinococcus granulosus, and Echinococcus multilocularis) are known to exist in Montana wildlife. E. multilocularis has been documented in coyotes (Seesee et al., 1983; Seesee et al., 1993) and in foxes (Seesee et al., 1993) in Montana for many years. E. granulosus is found almost worldwide, and has been documented in Montana over the past decade or so.  E. multilocularis and E. granulosus differ in morphology and in the hosts that they infect.

Distribution

Echinococcosis is an emerging disease found in many parts of the world.

In Montana, E. granulosus occurs  wherever the definitive host (wolves) and the intermediate host (ungulates) overlap. Foreyt et al., 2009 describes the prevalence of E. granulosus in wolves (definitive host) and ungulates (intermediate host) in Idaho and Montana (Foreyt et al., 2009). E. multilocularis likely occurs in coyotes and foxes across the state.

Species affected

Echinococcus granulosus occurs in wildlife species, livestock, and humans. The adult form of the parasite is present in the intestine of canids, which are the definitive host (wolves, coyotes, domestic dogs). The larval form is present in an intermediate host, typically domestic sheep, wild cervids (particularly moose, caribou and elk and bighorn sheep), and humans.

There are at least nine strains of E. granulosus that have adapted to different hosts, and in most cases occupy a wide geographical area. There are pastoral and sylvatic forms of the disease affecting domestic and wild animals, respectively. The sylvatic form has been reported in sheep, jackals, hyenas, warthogs, bush pigs, zebra, buffalo, wildebeest, and lions in Africa and moose, elk, caribou, white-tailed deer, wolf, coyote, and feral dogs in North America and Eurasia (Michigan, 2010).

There are two biotypes of E. granulosus in North America. The northern biotype, which has a canine definitive host and a cervid intermediate host, is thought to be the biotype found in Montana wolves. This biotype has been reported in moose, elk, caribou, white-tailed deer, wolves, coyote, and dogs in North America and Eurasia. In the Upper Peninsula of Michigan, a deer/coyote and a moose/wolf cycle has been observed. Cervid infections with this biotype primarily result in the development of cysts in the lungs, which often rupture and resolve with expulsion (Meltzer et al., 1956; Wilson et al., 1968). The second biotype (domestic biotype) is typically found in domestic dogs and domestic ungulates, especially sheep in the southwestern United States (Arizona, California, New Mexico, and Utah) (Foreyt et al., 2009).

The definitive hosts for Echinococcus multilocularis are arctic and red foxes, coyotes, sometimes domestic dogs and cats. Rodents (voles, mice, muskrats) are the intermediate hosts for E. multilocularis. Humans can rarely act as intermediate hosts for this parasite.

Transmission

The life cycles of E. granulosus and E. multilocularis are very similar, but with differences in host species and larval growth characteristics. Both parasites require two hosts to complete their life cycle:  a definitive host in which the adult parasites develop in the intestinal tract, and an intermediate host in which immature stages of the parasite encyst in organs.

In North America, the life cycle of the northern biotype of E. granulosus requires a definitive carnivore host (wolf, coyote, or dog) and an intermediate herbivore host (moose, elk, deer, caribou). E. multilocularis uses a carnivore definitive host such as a fox or coyote, and a rodent intermediate host.

The adult tapeworm is very small, usually measuring 3 to 6 mm in total length and resides in the small intestine of the definitive host. Large numbers of tapeworm eggs are shed in the fecal material of the definitive host, usually beginning about 1 month after infection. Eggs can survive at least 1 year in the environment, but arevulnerable to high temperatures, desiccation, and freezing, and may only survive for a couple hours in such conditions. Egg shedding in the definitive host may be cyclical and each worm can produce up to 1000 eggs every 10 days for up to 2 years. Each egg contains an embryo that serves as the infective stage. When the eggs are voided from the canid definitive host, they contaminate vegetation and may be ingested by the cervid intermediate host. Humans can be infected by ingestion of eggs acquired from contaminated food or water, from handling live canids or pelts from dead canids, or by handling canid fecal material (Miller et al, 2003; Michigan, 2010).

Once ingested bythe cervid intermediate host, the eggs hatch and release tiny hooks once they reach the small intestine. The embryo burrows through the wall of the intestine and enters the bloodstream, eventually lodging in an organ (liver, lungs, kidneys, brain, or bone marrow), with the lungs being the most common site. The same process can occur in human infection, with the liver being the most common destination (Michigan, 2010).

Once the larvae reach the organ of the intermediate host, they form a cyst, often referred to as a hydatid cyst. The cysts are round, thick walled, fluid-filled, and range in size from 2 to 30 cm in diameter. Development of these cysts is slow as the parasite is adapted to the long-lived intermediate host with larval tapeworms (protoscolices) developing in 1 to 2 years (Michigan, 2010).

The canid definitive host is infected by ingesting the intermediate host organ that contains the hydatid cyst. One cyst may contain hundreds to tens of thousands of protoscolices.. Following ingestion, the protoscolices develop into adult tapeworms which eventually produce eggs to complete the life cycle (Michigan, 2010). 

Signs

Infection with Echinococcus generally causes little or no harm to the definitive host. Impacts of infection of intermediate hosts with encysted larval stages of the parasite can be highly variable and  depends on the location and size of the cysts and intensity of the infection. Most hydatid cysts in cervid intermediate hosts reside in the lung, but they may also be found in the liver . Often, larval cysts are small and few, causing minimal adverse impacts on the health of the intermediate host. The structure of the cyst forms a tissue/host barrier enabling the cysts to be “walled off”, preventing further spread. Larval cysts may cause problems in host tissue if they are particularly large or numerous. Compression of lung tissue by large or numerous cysts may cause debilitation due to the animal’s reduced ability to breathe (Miller et al, 2003) . Occasionally larvae localize in the animal’s kidney, spleen, or brain tissue where their effects are more severe and often fatal (Michigan, 2010).

E. multilocularis infection may be more dangerous than E. granulosus as the larval cysts grow rapidly and bud externally acting very much like an invasive cancer. Unlike the cysts of E. granulosus, the structure of the wall of  E. multilocularis cysts does not form a tissue/host barrier, and they can further invade tissues via the lymph or blood. E. multilocularis severely debilitates and often kills its rodent intermediate host (Miller et al, 2003) .

Public Health Concerns

There is some risk of humans becoming infected with Echinococcus granulosus or Echinococcus multilocularis. The northern biotype of Echinococcus granulosus, which cycles between canids like wolves, and ungulates like deer and elk, is present in Montana. While the risk of human infection resulting from the wolf-ungulate cycle in Montana is theoretically possible, it is highly unlikely (Foreyt et al. 2009). When human infection does occur with this particular biotype it is considered by some experts to be relatively benign (Rausch, 2003). Human cases caused by the domestic biotype of E. granulosus are also very uncommon, but do occur, especially in high-risk groups having close contact with sheepherding dogs in the southwestern United States (Arizona, California, New Mexico, Utah) (Foreyt et al., 2009). Utah has been reported to have the highest number of surgical human cases in the United States. From 1944-1994, 45 surgical cases were reported. To become infected, a human must ingest parasite eggs, which are passed with the feces of an infected definitive host (typically a canid such as a wolf, coyote or fox). Eggs could be ingested while consuming vegetation or drinking water that has been contaminated with infected feces. Humans could also become infected after handling contaminated canine scat or fur, and then transferring eggs to the mouth by touching the face or eating before adequate hand washing.

Echinococcus infection in humans can lead to development of cysts in organs such as the lungs, liver or brain, just as it does with other intermediate hosts. If an Echinococcus infection in a human occurs, it can result in a variety of symptoms that depend on the site of the infection (e.g. liver, lungs, or brain) (CDC).

Cysts may develop over prolonged periods of time (10-15 years) before any clinical signs are evident. Treatment may involve surgical removal of cysts and treatment with anthelmintic medications.

Prevention of E. granulosus in humans can be accomplished primarily through education and proper hygiene. When handling live canids, their feces, pelts, or carcasses, wear disposable gloves, a face mask, and practice good hygiene. There are no precautions that need to be taken when handling tissue of the intermediate hosts as the lung cysts are not infective to humans (Michigan, 2010Stitt, 2011). Domestic dogs can serve as reservoirs for Echinococcus infection and should not be fed carcasses or allowed to scavenge from infected game mammals. Dogs could become infected and serve as a source of infection to humans. Dog owners should discuss their dog’s risk of infection with their veterinarian to ensure that the dog is on an appropriate de-worming protocol. DO NOT FEED TISSUES CONTAINING cysts TO DOGS (Miller et al, 2003).

Is it safe to eat the meat?

Humans do not become infected by handling or eating hydatid cysts from the intermediate host (domestic sheep, deer, elk, moose). For aesthetic reasons, cysts should be removed prior to consumption. Meat of animals infected by these parasites should not be fed to dogs since dogs can be hosts for the adult tapeworms. If you harvest an animal and are unsure whether it is safe to eat, contact your local FWP staff for guidance soon after the animal is harvested.

Citations

Foreyt, W.J., M.L. Drew, M. Atkinson, and D. Mccauley. 2009. Echinococcus granulosus in gray wolves and ungulates in Idaho and Montana, USA. Journal of Wildlife Diseases 45:1208-1212.

Michigan Wildlife Disease Manual. 2010. Michigan Department of Natural Resources.Website

Miller, M.J.R., R.D. Dawson, and H. Schwantje. 2003. Manual of Common Diseases and Parasites of Wildlife in Northern British Columbia. University of Northern British Columbia.

Rausch, R.L. 2003. Cystic echinococcosis in the Arctic and Sub-Arctic. Parasitology 127:S73-S85.

Seesee, F.M., M.C. Sterner, and D.E. Worley. 1983. Helminths of the coyote (Canis latrans Say) in Montana. Journal of Wildlife Diseases 19(1):54-55.

Seesee, F.M., M.C. Sterner, and D.E. Worley. 1993. Echinococcus multilocularis: (Cestoda: Taeniidae) in Montana: Additional locality records in foxes and coyotes. Proc. Montana Academy of Sciences. Pp. 9-14.

Stitt, Tyler. 2011. Diseases You Can Get From Wildlife - A Field-guide for Hunters, Trappers, Anglers and Biologists. British Columbia Ministry of the Environment.

Wilson, J.F., A.C. Diddams, and R.L. Rausch. 1968. Cystic hydatid disease in Alaska: a review of 101 autochthonous cases of Echinococcus granulosus infection. American Review Respiratory Disease 98(1):1-15.